The present invention relates to the art of magnetic resonance cine imaging. It finds particular application in conjunction with black blood cine imaging of the heart and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with angiography, circulatory, and other examinations in which a flowing fluid is to be displayed dark.
Cine images have commonly been acquired using field echoes. Field echoes permit a rapid repetition rate, e.g. up to 64 sequence repetitions per second. In cardiac cine imaging, this enabled a view of data for each of 64 images to be collected in a single cardiac cycle. However, in the images from field echo sequences, the blood appears bright. The bright area of the image corresponding to blood tends to cause artifacts and ghosting. The problem of ghosting in gradient echo cine imaging is in part due to changes in the position of the heart due to respiration and cardiac beating from sequence repetition to sequence repetition.
An additional disadvantage of gradient echo imaging is a blurring of the interface between the heart tissue, and the blood. More specifically, a loss of contrast occurs between slow moving blood and the myocardium when the slow moving blood remains within the imaging plane long enough to become saturated. This typically occurs, but not exclusively, in the apex of the heart, making it difficult to determine myocardial mass, wall thickness, and chamber volumes accurately.
It is commonplace in angiography studies to saturate the blood tissue such that it appears dark in the resultant image. However, the saturation techniques used in angiography requires large RF pulses and large spoiler gradients. These large RF pulses and spoiling gradients not only require a large amount of power, but also require a relatively long duration. The duration needed to achieve saturation in angiography sequences with large spoilers is too long for effective cine examinations.
In a technique described in "Cineangiography of the Heart in a Single Breath Hold with Segmented TurboFLASH Sequence", Radiology, Vol. 178, pp. 357-360, Atkinson and Edelman (RSNA, 1991), limited pre-saturation was incorporated into a cine sequence. More specifically, the Atkinson and Edelman technique generated cine images along the long axis of the heart, i.e. generally vertically in a standing patient. This technique pre-saturated the atria to saturate blood flowing into the heart. This caused the blood in the resultant image to be dark during diastole when the saturated blood flowed in from the atria. However, during systole, when the heart is contracting and hence, in the most clinically interesting portion of its cycle, this technique results in blood that is neither bright nor dark. Rather, the blood is gray, very close in intensity to the gray scale of the surrounding myocardium. Thus, this technique does not provide good images for measuring cardiac volume during end diastole or end systole. Moreover, the pre-saturation causes a dark stripe to appear across the image, totally obliterating the signal from the atria.
Another disadvantage of the Atkinson technique is that Atkinson uses very large spoiling gradients to prevent signal from the pre-saturation slab from interfering with the desired image. These large spoiler gradients significantly decrease temporal resolution, i.e. lengthen the sequence time.
Dark blood images can also be obtained using spin echo techniques. Spin echo cines are obtained by acquiring multiple acquisitions with varying delays after the R-wave. These acquisitions are then sorted and recombined based upon their temporal order. Although spin echo images generally have dark blood and show good anatomical detail, they suffer from a very low temporal resolution, typically 6-12 views per cardiac cycle. A one-frame image typically requires about 4 minutes and a six-frame cine would require more than 24 minutes. This is much slower than gradient echo cines in which 20-64 views can be acquired in as little as one minute.
Further, spin echo images of the heart also suffer from the same type of ghosting as gradient echo cines. The ghosting is particularly apparent during diastole when the blood is flowing more slowly or not at all, depending on the slice location.
The present invention contemplates a new and improved cine technique which overcomes the above-referenced problems and others to provide dark blood cine images with high temporal resolution.